Added a configuration option to suffix/prefix the prompt

[hallzy]

Now, you can add a prefix and/or suffix to your git-radar prompt that will only show if you are in a git repo.

Note: The PROMPT_FORMAT line was changed to remove the leading space in the string. With it in, if you define a prefix a space will still be there even if the user may not want it. So it can be overridden by the user with one of the new variables, but by default, the prefix still adds that space anyways.